1. Field of the Invention
This invention relates to switched reluctance motors.
2. Description of Related Art
A form of switched electric reluctance motor is proposed in U.S. Pat. No. 5,043,618. The motor disclosed in that patent has two poles per pole group, and stator and rotor pole arcs of about 30.degree.. An example is illustrated in FIG. 1 of the drawings. FIG. 2 illustrates the static torque curve, which is derived from the torque developed by the motor with a constant excitation current in the windings, as a function of the angle .theta. of rotation of the rotor. Such curves are characteristic of doubly salient pole motors and can be calculated or measured.
Positive torque is assumed to drive the motor in the forward, clockwise, direction so that the angle of rotation .theta. is increasing positively in FIG. 2 in normal operation. At low speed the winding carries current throughout the range of the angle of rotation .theta. corresponding to the positive torque, from .theta.=-45.degree. to .theta.=0.degree..
It will be appreciated with reference to FIG. 2 that the motor will only start from rest in the forward direction if the rotor is in a position corresponding to a region of positive torque. On the other hand, if the coil is switched across a d.c. voltage supply when the rotor is in a region corresponding to negative torque, the resulting current in the winding will cause the motor to move in the reverse direction. It will be further appreciated that there are two positions in each torque cycle at which no torque will be developed, e.g. .theta.=0.degree. and .theta.=-45.degree. in FIG. 2. A rotor angle of .theta.=0.degree. corresponds to a stable condition because if the rotor is displaced from this position in either direction, with current flowing in the winding, the torque will restore the rotor to the initial position. On the other hand, position .theta.=-45.degree. corresponds to an unstable position because displacement of the rotor from the position in either direction will cause the rotor to move away from the initial position.
The rotor 1 in FIG. 1 is in a stable position of zero torque in relation to the stator 2. Thus, in this position, the motor will fail to start when current is fed to the windings.
In the motor of FIG. 3 the rotor is in the unstable position of zero torque with the inter-pole axes of the rotor 1 aligned with the axes of the stator poles.
Thus, there are regions in the rotor cycle in which the above motor will not start and regions in which there will be uncertainty as to at least the initial direction of rotation of the rotor.